Method of reducing a blocking artifact when coding moving picture

ABSTRACT

A method of coding a moving picture is provided that reduces blocking artifacts. The method can include defining a plurality of defining pixels S 0 , S 1 , and S 2 , which are centered around a block boundary. If a default mode is selected then frequency information of the surroundings of the block boundary is obtained. A magnitude of a discontinuous component in a frequency domain belonging to the block boundary is adjusted based on a magnitude of a corresponding discontinuous component selected from a pixel contained entirely within a block adjacent the block boundary. The frequency domain adjustment is then applied to a spatial domain. Or, a DC offset mode can be selected to reduce blocking artifacts in smooth regions where there is little motion.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.09/506,728, filed on Feb. 18, 2000, now U.S. Pat. No. 7,239,755 which isa continuation of U.S. application Ser. No. 09/010,446, filed date Jan.22, 1998 now U.S. Pat. No. 6,028,967, which claims the benefit of aforeign priority application filed in KOREA on Jul. 30, 1997, as SerialNo. 36231/1997. This application claims priority to all of theseapplications, and all of these applications are incorporated byreference.

TECHNICAL FIELD

The present invention relates to a moving picture process, and inparticular to a method for processing blocks of a moving picture toincrease a compression ratio and to improve coding efficiency.

BACKGROUND

To efficiently compress a time variable video sequence, redundancy inthe temporal domain as well as in the two dimensional spatial domainmust be reduced. MPEG uses a discrete cosine transform (DCT) to reducethe redundancy in the two dimensional spatial domain and a motioncompensation method to reduce the redundancy in the temporal domain.

The DCT is a method of reducing the correlativity between data through atwo dimensional spatial transformation. Each block in a picture isspatially transformed using the DCT after the picture is divided intoblocks. Data that has been spatially transformed tends to be driven to acertain direction. Only a group of the data driven in the certaindirection is quantized and transmitted.

Pictures, which are consecutive in the temporal domain, form motions ofa human being or an object at the center of the frame. This property isused to reduce the redundancy of the temporal domain in the motioncompensation method. A volume of data to be transmitted can be minimizedby taking out a similar region from the preceding picture to fill acorresponding region, which has not been changed (or has very littlechange), in the present picture. The operation of finding the mostsimilar blocks between pictures is called a motion estimation. Thedisplacement representing a degree of motion is called a motion vector.MPEG uses a motion compensation-DCT method so that the two methodscombine.

When a compression technique is combined with a DCT algorithm, the DCTtransform is usually performed after input data is sampled in a unitsize of 8×8, and the transform coefficients are quantized with respectto a visual property using quantization values from a quantizationtable. Then, the data is compressed through a run length coding (RLC).The data processed with the DCT is converted from a spatial domain to afrequency domain and compressed through the quantization with respect tothe visual property of human beings, not to be visually recognized. Forexample, since eyes of human beings are insensitive to a high frequency,a high frequency coefficient is quantized in a large step size. Thus, aquantization table is made according to external parameters, such as adisplay characteristic, watching distance, and noise, to perform anappropriate quantization.

For the quantized data, the data having a relatively high frequency iscoded with a short code word. The quantized data having a low frequencyis coded with a long code word. Thus, the data is finally compressed.

In processing a moving picture as discussed above, blocks areindividually processed to maximize the compression ratio and codingefficiency. However, the individual process causes blocking artifactsthat disturb the eyes of human beings at boundaries between blocks.

Accordingly, various methods for reducing a blocking artifact in acoding system, which individually processes blocks, are presented. Forexample, attempts to reduce the blocking artifact by changing processesof coding and decoding have been implemented. However, this method ofchanging the processes of coding and decoding increases the amount ofbits to be transmitted.

Another method for reducing the blocking artifact is based on the theoryof projection onto convex sets (POCS). However, this method is appliedto only a still picture because of an iteration structure andconvergence time.

The blocking artifact is a serious problem in a low transmit rate movingpicture compression. Since a real-time operation is necessary in codingand decoding a moving picture, it is difficult to reduce the blockingartifact with a small operation capacity.

Consequently, the related art methods involve various problems anddisadvantages when reducing a blocking artifact created in coding amoving picture. A calculation for performing an algorithm iscomplicated, and the calculation amount and time become correspondinglylarge. Further, the blocking artifacts are not reduced in either complexregions or smooth regions in a picture. In addition, the amount of bitsto be transmitted increases.

The above references are incorporated by reference herein whereappropriate for appropriate teachings of additional or alternativedetails, features and/or technical background.

SUMMARY

An object of the present invention is to provide a method for reducing ablocking artifact appearing when coding a moving picture thatsubstantially obviates one or more of the limitations and disadvantagesof the related art.

Another object of the present invention is to provide an MPEG-4 videocoding method that reduces a blocking artifact in a real-time movingpicture using a frequency property around boundaries between blocks.

A further object of the present invention is to provide a method forreducing a blocking artifact that increases a compression ration andincreases a coding efficiency.

To achieve these and other advantages in whole or in parts, and inaccordance with the purpose of the present invention as embodied andbroadly described, a blocking artifact reduction method includesdefining pixels centered around a block boundary and setting a defaultmode. Frequency information of the surroundings of the block boundary isobtained for each pixel using a 4-point kernel. A magnitude of adiscontinuous component that belongs to the block boundary is adjustedin a frequency domain to a minimum value of a magnitude of adiscontinuous component that belongs to the surrounding of the blockboundary. The adjusting operation is then applied to a spatial domain.In addition, a DC offset mode is established, and in the DC offset modethe blocking artifact is also reduced, for example, in a smooth regionwhere there is little motion.

Additional advantages, objects, and features of the invention will beset forth in part in the description which follows and in part willbecome apparent to those having ordinary skill in the art uponexamination of the following or may be learned from practice of theinvention. The objects and advantages of the invention may be realizedand attained as particularly pointed out in the appended claims.

DESCRIPTION OF DRAWINGS

The invention will be described in detail with reference to thefollowing drawings in which like reference numerals refer to likeelements wherein:

FIG. 1 is a diagram that illustrates horizontal and vertical blockboundaries;

FIG. 2 is a diagram that illustrates a 4-point DCT kernel; and,

FIG. 3 is a flow chart that illustrates a preferred embodiment of amethod that reduces a blocking artifact when coding a moving pictureaccording to the present invention.

DETAILED DESCRIPTION

Reference will now be made to preferred embodiments of the presentinvention, examples of which are illustrated in the accompanyingdrawings. FIG. 1 illustrates typical horizontal and vertical blockboundaries.

As shown in FIG. 1, in the dimensional image formed with respective fourpoints of S₀, S₁, and S₂ located around the block boundary, S₁ and S₂are individually processed with a block-unit compression method. Thus,S₁ and S₂ are not influenced by the blocking artifact. However, S₀ islocated across a block boundary. Thus, S₀ is directly influenced by theblocking artifact. The blocking artifact appears at the boundary betweenfixed block patterns in the form of a line of discontinuity.

Preferred embodiments of the present invention use, for example, afrequency property to preserve complex regions at block boundaries. Thefrequency property around the boundary is preferably obtained by using a4-point DCT kernel, which can be easily calculated. However, the presentinvention is not intended to be limited to this. In this case, thecomplex region at a block boundary can be effectively processed byextending the smoothness of a picture from a frequency domain to aspatial domain.

As shown in FIG. 1, S₀ is located across the block boundary. Thus, S₀ isdirectly influenced by the blocking artifact. To reduce the blockingartifact from S₀, a first preferred embodiment of the present inventionuses frequency information in S₁ and S₂. The blocking artifact can beremoved from S₀ by replacing the frequency component in S₀, which isinfluenced by the blocking artifact, with the frequency components of S₁and S₂. In other words, S₀ contains a discontinuity. However, S₁ and S₂,which are completely included inside respective blocks, are not relatedto the discontinuity. Since S₁ and S₂ are not involved with thediscontinuity at a block boundary, S₁ and S₂ can accurately representfeatures of the respective neighboring blocks.

When images change smoothly, image features of S₀, S₁ and S₂ aresimilar. This means that frequency domains of S₀, S₁ and S₂ have similarfeatures. The preferred embodiments use a DCT, or the like as afrequency analysis tool. DCT is widely used in an image compressiontechnique.

FIG. 2 is a diagram illustrating a 4-point DCT basis. As shown in FIG.2, the 4-point DCT kernel basis has symmetric and anti-symmetricproperties around the center of 4 points. In FIG. 2, a_(0,0), a_(1,0),a_(2,0), and a_(3,0) are defined as the 4-point DCT coefficients of S₀.Although both a_(2,0), and a_(3,0) are high frequency components,a_(2,0) is symmetric, and a_(3,0) is anti-symmetric around the center.

The center of S₀ is located at a block boundary as shown in FIG. 1.Thus, a factor directly affecting the block discontinuity is not thesymmetric component but the anti-symmetric component. The magnitude ofa_(3,0) in a frequency domain is thus adjusted based on theanti-symmetric component being a major factor affecting thediscontinuity. Accordingly, the proper adjustment of a_(3,0) is directlyrelated to the reduction of block discontinuity in the spatial domain.Reduction of the block discontinuity will now be described.

In a first preferred embodiment, the magnitude of a_(3,0) is replacedwith the minimum value of the magnitudes of a_(3,1) and a_(3,2), whichare contained in a single block in an area surrounding a block boundary.By doing this, a large blocking artifact that appears when one side ofthe block boundary to be processed is smooth can be reduced. For acomplex image where both S1 and S2 are the objects of motion (i.e., allthe values of the magnitudes of a_(3,0), a_(3,1) and a_(3,2) are large),there is little influence on the block boundary.

A method for reducing a blocking artifact in a default mode is asfollows:v ₃ ′=v ₃ −d;v ₄ ′=v ₄ +d; andd=CLIP(c ₂(a _(3,0) −a _(3,0))//c ₃,0,(v ₃ −v ₄)/2)*δ(|a _(3,0) |<QP).

In the method, a_(3,0)′=SIGN(a_(3,0))*MIN(|a_(3,0)|, |a_(3,1)|,|a_(3,2)|), and q is t component of DCT kernel. The condition|a_(3,0)|<QP is used to count the influence of the quantizationparameter on the blocking artifact. The |a_(3,0)|<QP condition alsoprevents over-smoothing when the blocking artifact is not very serious.The clipping operation on the compensated value prevents the directionof the gradient at the boundary from being large or changed in anopposite direction. The boundary pixel values, v₃ and v₄, are replacedwith v₃′ and v₄′. QP is the quantization parameter of the macroblockwhere v₄ belongs. Values, c₁, c₂, and c₃ are kernel constants used inthe 4-point DCT. To simplify an equation according to a first preferredembodiment of the present invention, the values of c₁ and c₂ areapproximated to an integer, and the value of c₃ is approximated to amultiple of 2. The values of a₁, a₂, and a₃ are evaluated from thesimple inner product of the DCT kernel and pixels, S₀, S₁, and S₃.a _(3,0)=([c ₁ −c ₂ c ₂ −c ₁ ]*[v ₂ v ₃ v ₄ v ₅]^(T))/c ₃a _(3,1)=([c ₁ −c ₂ c ₂ −c ₁ ]*[v ₀ v ₁ v ₂ v ₃]^(T))/c ₃a _(3,2)=([c ₁ −c ₂ c ₂ −c ₁ ]*[v ₄ v ₅ v ₆ v ₇]^(T))/c ₃

Such processes are performed in both horizontal and vertical blockboundaries. In this manner, the blocking artifacts in the whole framecan be reduced.

The first embodiment reduces a blocking artifact in the default mode.However, in the default mode, only the boundary pixel values, v₃ and a₄,are compensated. Thus, the default mode is not sufficient to reduce theblocking artifact in a very smooth region, such as a setting in apicture.

To reduce the blocking artifact in the smooth region, a second preferredembodiment of a method for reducing blocking artifacts in a movingpicture according to the present invention includes a DC offset mode.The method in the DC offset mode is as follows:v ₃ ′=v ₃ −d;v ₄ ′=v ₄ +d;v ₂ ′=v ₂ −d ₂;v ₅ ′=v ₅ +d ₂;v ₁ ′=v ₁ −d ₃; andv ₆ ′=v ₆ +d ₃.

In the second preferred embodiment,d ₁=(3(v ₃ −v ₄)/8)*δ(|a _(3,0) |<QP),d ₂=(3(v ₃ −v ₄)/16)*δ(|a _(3,0) |<QP), andd ₃=(3(v ₃ −v ₄)/32)*δ(|a _(3,0) |<QP).

The blocking artifact in the region where there is little motion, orwhich is a very small setting, is reduced through the above-describedmethod or the like in the DC offset mode. An appropriate mode betweenthe DC offset mode and default mode can be determined using thefollowing conditional expression:If (v ₀ ==v ₁&&v ₃ ==v ₂&&v ₂ ==v ₃&&v ₄ ==v ₅&&v ₅ ==v ₆&&v ₆ ==v ₇)

DC offset mode is applied; Else Default mode is applied.

When the DC offset mode or the default mode is selected according to theabove conditional expression, the blocking artifacts are reduced in eachmode. After determining the proper mode between the DC offset mode andthe default mode, the block discontinuity at the boundary is compensatedto form a consecutive line, which reduces the blocking artifact. In thesecond preferred embodiment, the DC offset mode and the default mode areset using S₀, S₁ and S₂. However, the present invention is not intendedto be limited to this. Alternative sets of points or the like can beused.

An exemplary method for reducing a blocking artifact when coding amoving picture, according to the second preferred embodiment of thepresent invention, is described with reference to the flow chart shownin FIG. 3.

After beginning in FIG. 3, control continues to step 101. In step 101, aplurality of pixels, S0, S1, and S2 are defined centering around a blockboundary. From step 101, control continues to step 102. In step 102, ifa mode is selected, a default mode is set, and control continues to step103.

In step 103, frequency information of the surroundings of the blockboundary for each pixel is obtained using, for example, the 4-point DCTkernel. From step 103, control continues to step 104. In step 104, themagnitude of discontinuous component belonging to the block boundary isreplaced with the minimum magnitude of the discontinuous componentsbelonging to the surroundings of the block boundary in the frequencydomain. From step 104, control continues to step 105, where theadjusting operation is applied to the spatial domain. The default modeis effective in reducing the blocking artifact in a complex region of apicture. However, the default mode is less successful in a smooth regionsuch as a setting in a picture.

Therefore, in a smooth region it is necessary to reduce the blockingartifact in another mode, the DC offset mode. In step 106, the DC offsetmode is established. From step 106, control continues to step 107. Instep 107, the blocking artifact in the region where there is littlemotion, such as a setting, is reduced. From step 107, the process ends.Thus, the overall blocking artifacts can be reduced according to thepreferred embodiments.

As described above, the blocking artifact reduction methods according tothe preferred embodiments of the present invention have variousadvantages and effects. The blocking artifact is more easily andeffectively reduced using features of the frequency domain. Thepreferred embodiments provide a visually finer quality of a picture byreducing the blocking artifacts in both the complex and smooth regions.Further, calculations are simple. Accordingly, the amount of bits doesnot increase.

The foregoing embodiments are merely exemplary and are not to beconstrued as limiting the present invention. The present teaching can bereadily applied to other types of apparatuses. The description of thepresent invention is intended to be illustrative, and not to limit thescope of the claims. Many alternatives, modifications, and variationswill be apparent to those skilled in the art.

1. A method of reducing blocking artifacts when coding a pictureincluding a collection of points, the method comprising: dividing thepicture into blocks that each include multiple points, wherein thepoints of a first block are separated from points of a neighboringsecond block by a block boundary; selecting a correction mode usinginformation from the first block and information from the second block,wherein the correction mode is selected from at least two correctionmodes; and when a first correction mode is selected, modifying the firstand second points and also modifying a third point adjacent to the firstpoint and separated from the second point by no points other than thefirst point, and a fourth point adjacent to the second point andseparated from the first point by no points other than the second point.2. The method of claim 1, wherein the first point is modified in a firstway when the first correction mode is selected, the method furthercomprising modifying the first point in a second, different way when asecond correction mode is selected.
 3. The method of claim 1, furthercomprising, when the first and second points are to be modified, alsomodifying a fifth point adjacent to the third point and separated fromthe first point by no points other than the third point, and a sixthpoint adjacent to the fourth point and separated from the second pointby no points other than the fourth point.
 4. The method of claim 3,wherein the first through sixth points define a line segment centered onthe block boundary.
 5. The method of claim 4, wherein the line segmentcomprises a horizontal line segment.
 6. The method of claim 4, whereinthe line segment comprises a vertical line segment.
 7. The method ofclaim 3, wherein the first, third and fifth points are modified bydifferent amounts.
 8. The method of claim 3, wherein determining whetherthe first and second points are to be modified comprises determiningwhether a function of values of the first and second points exceeds athreshold value and determining that the first and second points are tobe modified only when the function does not exceed the threshold value.9. The method of claim 3, wherein determining whether the first andsecond points are to be modified comprises determining whether afunction of values of the first, second, third and fourth points exceedsa threshold value and determining that the first and second points areto be modified only when the function does not exceed the thresholdvalue.
 10. The method of claim 2, wherein the first through fourthpoints define a line segment centered on the block boundary.
 11. Themethod of claim 10, wherein the line segment comprises a horizontal linesegment.
 12. The method of claim 10, wherein the line segment comprisesa vertical line segment.
 13. The method of claim 9, wherein the firstand third points are modified by different amounts.
 14. The method ofclaim 9, wherein determining whether the first and second points are tobe modified comprises determining whether a function of values of thefirst and second points exceeds a threshold value and determining thatthe first and second points are to be modified only when the functiondoes not exceed the threshold value.
 15. The method of claim 9, whereindetermining whether the first and second points are to be modifiedcomprises determining whether a function of values of the first, second,third and fourth points exceeds a threshold value and determining thatthe first and second points are to be modified only when the functiondoes not exceed the threshold value.